Ink jet printing is a type of printing that recreates a digital image by propelling drops of ink onto paper, plastic, or other substrates. There are two main technologies in use: continuous (CIJ) and Drop-on-demand (DOD) inkjet.
In continuous inkjet technology, a high-pressure pump directs the liquid solution of ink and fast drying solvent from a reservoir through a gunbody and a microscopic nozzle, creating a continuous stream of ink drops via the Plateau-Rayleigh instability. A piezoelectric crystal creates an acoustic wave as it vibrates within the gunbody and causes the stream of liquid to break into drops at regular intervals. The ink drops are subjected to an electrostatic field created by a charging electrode as they form; the field varies according to the degree of drop deflection desired. This results in a controlled, variable electrostatic charge on each drop. Charged drops are separated by one or more uncharged “guard drops” to minimize electrostatic repulsion between neighboring drops. The charged drops pass through an electrostatic field and are directed (deflected) by electrostatic deflection plates to print on the receptor material (substrate), or allowed to continue on undeflected to a collection gutter for re-use. The more highly charged drops are deflected to a greater degree. Only a small fraction of the drops is used to print, the majority being recycled. The ink system requires active solvent regulation to counter solvent evaporation during the time of flight (time between nozzle ejection and gutter recycling), and from the venting process whereby gas that is drawn into the gutter along with the unused drops is vented from the reservoir. Viscosity is monitored and a solvent (or solvent blend) is added to counteract solvent loss.
Drop-on-demand (DOD) may be divided into low resolution DOD printers using electro valves in order to eject comparatively big drops of inks on printed substrates, or high resolution DOD printers, may eject very small drops of ink by means of using either a thermal DOD and piezoelectric DOD method of discharging the drop.
In the thermal inkjet process, the print cartridges contain a series of tiny chambers, each containing a heater. To eject a drop from each chamber, a pulse of current is passed through the heating element causing a rapid vaporization of the ink in the chamber to form a bubble, which causes a large pressure increase, propelling a drop of ink onto the paper. The ink's surface tension, as well as the condensation and thus contraction of the vapor bubble, pulls a further charge of ink into the chamber through a narrow channel attached to an ink reservoir. The inks used are usually water-based and use either pigments or dyes as the colorant. The inks used must have a volatile component to form the vapor bubble, otherwise drop ejection cannot occur.
Piezoelectric DOD use a piezoelectric material in an ink-filled chamber behind each nozzle instead of a heating element. When a voltage is applied, the piezoelectric material changes shape, which generates a pressure pulse in the fluid forcing a drop of ink from the nozzle. A DOD process uses software that directs the heads to apply between zero to eight drops of ink per dot, only where needed.
High resolution printers, alongside the office applications, are also being used in some applications of industrial coding and marking. Thermal Ink Jet more often is used in cartridge based printers mostly for smaller imprints, for example in pharmaceutical industry. Piezoelectric printheads of companies like Spectra or Xaar have been successfully used for high resolution case coding industrial printers.
All DOD printers share one feature in common: the discharged drops of ink have longer drying time compared to CIJ technology when applied on non porous substrate. The reason being usage of fast drying solvent, which is well accepted by CIJ technology designed with fast drying solvent in mind, but which usage needs to be limited in DOD technology in general and high resolution DOD in particular. That is because fast drying inks would cause the dry back on the nozzles. In most of known applications the drying time of high resolution DOD printers' imprints on non porous substrates would be at least twice and usually well over three times as long as that of CIJ. This is a disadvantage in certain industrial coding applications, for instance very fast production lines where drying time of few seconds may expose the still wet (not dried) imprint for damage when it gets in contact with other objects.
Another disadvantage of high resolution DOD technology is limited drop energy, which requires the substrate to be guided very evenly and closely to printing nozzles. This also proves to be disadvantageous for some industrial applications. For example when coded surface is not flat, it cannot be guided very close to nozzles.
CIJ technology also proves to have inherent limitations. So far CIJ has not been successfully used for high resolution imprints due to the fact that it needs certain drop size in order to work well. The other well-known disadvantage of CIJ technology is high usage of solvent. This causes not only high costs of supplies, but also may be hazardous for operators and the environment, since most efficient solvents are poisonous, such as the widely used MEK (Methyl Ethyl Ketone).
The following documents illustrate various improvements to the ink jet printing technology.
An article “Double-shot inkjet printing of donor-acceptor-type organic charge-transfer complexes: Wet/nonwet definition and its use for contact engineering” by T. Hasegawa et al (Thin Solid Films 518 (2010) pp. 3988-3991) presents a double-shot inkjet printing (DS-IJP) technique, wherein two kinds of picoliter-scale ink drops including soluble component donor (e.g. tetrathiafulvalene, TTF) and acceptor (e.g. tetracyanoquinodimethane, TCNQ) molecules are individually deposited at an identical position on the substrate surfaces to form hardly soluble metal compound films of TTF-TCNQ. The technique utilizes the wet/nonwet surface modification to confine the intermixed drops of individually printed donor and acceptor inks in a predefined area, which results in the picoliter-scale instantaneous complex formation.
A U.S. Pat. No. 7,429,100 presents a method and a device for increasing the number of ink drops in an ink drop jet of a continuously operating inkjet printer, wherein ink drops of at least two separately produced ink drop jets are combined into one ink drop jet, so that the combined ink drop jet fully encloses the separate ink drops of the corresponding separate ink drop jets and therefore has a number of ink drops equal to the sum of the numbers of ink drops in the individual stream. The drops from the individual streams do not collide with each other and are not combined with each other, but remain separate drops in the combined drop jet.
A US patent application US20050174407 presents a method for depositing solid materials, wherein a pair of inkjet printing devices eject ink drops respectively in a direction such that they coincide during flight, forming mixed drops which continue onwards towards a substrate, wherein the mixed drops are formed outside the printing head.
A Japanese patent application JP2010105163A discloses a nozzle plate that includes a plurality of nozzle holes that discharge liquids that combine in flight outside the nozzle plate.
A U.S. Pat. No. 8,092,003 presents systems and methods for digitally printing images onto substrates using digital inks and catalysts which initiate and/or accelerate curing of the inks on the substrates. The ink and catalyst are kept separate from each other while inside the heads of an inkjet printer and combine only after being discharged from the head, i.e. outside the head. This may cause problems in precise control of coalescence of the drops in flight outside the head and corresponding lack of precise control over drop placement on the printed object.
There are known various arrangements for altering the velocity of the drop exiting the printing head by using electrodes for affecting charged drops, as described e.g. in patent documents U.S. Pat. No. 3,657,599, US20110193908 or US20080074477.
The US patent application US20080074477 discloses a system for controlling drop volume in continuous ink-jet printer, wherein a succession of ink drops, all ejected from a single nozzle, are projected along a longitudinal trajectory at a target substrate. A group of drops is selected from the succession in the trajectory, and this group of drops is combined by electrostatically accelerating upstream drops of the group and/or decelerating downstream drops of the group to combine into a single drop.
German patent applications DE3416449 and DE350190 present CIJ printing heads comprising drop generators which generate a continuous stream of drops. The stream of drops is generated as a result of periodic pressure disturbances in the vicinity of the nozzles that decompose the emerging inkjets to drops which have the same size and are equally spaced. The majority of drops are collected by gutters and fed back to the reservoirs supplying ink to the drop generators, as common in the CIJ technology.
A Japanese patent application JPS5658874 presents a CIJ printing head comprising nozzles generating continuous streams of drops, which are equally spaced, wherein some of the drops are collected by gutters and only some of the drops reach the surface to be printed. The paths of drops are altered by a set of electrodes such that the path of one drop is altered to cross the path of another drop.
Due to substantial structural and technological differences between the CIJ and DOD technology print heads, these print heads are not compatible with each other and individual features are not transferrable between the technologies.
A U.S. Pat. No. 8,342,669 discloses an ink set comprising at least two inks, which can be mixed at any time (as listed: before jetting, during jetting, or after jetting). A particular embodiment specifies that the inks may be mixed or combined anywhere between exiting the ink jet head and the substrate, that is, anywhere in flight. After combination of the inks between the ink jetting device and the substrate, the drops of the inks may begin to react, that is polymerization of the vinyl monomers may begin and momentum of the drops may carry the drops to a desired location on the substrate. This has, however, the disadvantage, that it is difficult to control the parameters of coalescence of the drops, as it the surrounding outside the ink jetting device is variable.
A US patent application US2011/0181674 discloses an inkjet print head including a pressure chamber storing a first ink drawn in from a reservoir and transferring the first ink to a nozzle by a driving force of an actuator; and a damper disposed between the pressure chamber and the nozzle and allowing the first ink to be mixed with a second ink drawn through an ink flow path for the second ink. The disadvantage of that solution is that the mixed ink is in contact with the nozzle. This can lead to problems when the physicochemical parameters of the mixed ink do not allow for jetting of the mixed ink, or the mixed ink is not chemically stable and reactions occurring within the mixed ink cause the change of physicochemical parameters that do not allow for jetting of the mixed ink, or the reaction causes solidification of the mixed ink. In case the chemical reaction is initiated while mixing the ink components, any residue of the mixed ink which gets in contact with the nozzle and is not removed out of it by the stream of gas may cause the residue build up leading to blocking the nozzle during printing process.